Parking brake modulators are installed in pneumatic and electropneumatic brake systems. A parking brake modulator has valves by means of which a pneumatic pressure can be output and provided for releasing a parking brake. For this purpose, combined spring store/diaphragm brake cylinders are conventionally installed for actuating vehicle brakes. A combined spring store/diaphragm brake cylinder differs from a simple brake cylinder, which serves only to actuate a service brake, in that it has a spring store part for the parking brake in addition to a diaphragm part for the service brake, with the parking brake being engaged when the spring store part is in the deaerated state and being released when the spring store part is aerated if the diaphragm part of the spring store/diaphragm brake cylinder is not aerated.
Here, the terms “service brake” and “parking brake” refer to the service brake function and the parking brake function of one or more brakes. The service brake and the parking brake may be realized by the same brake if the brake has both functionalities, that is, a service brake function and a parking brake function.
During driving operation of the vehicle, the spring store parts of the combined spring store/diaphragm brake cylinder are aerated by means of the parking brake modulator, or an air pressure in the spring store parts of the spring store/diaphragm brake cylinder is held. To engage the parking brake, the spring store parts of the combined spring store/diaphragm brake cylinder can be deaerated by means of the parking brake modulator.
The parking brake modulator provides an output reservoir pressure for the trailer vehicle at an outlet. If the brakes of the trailer vehicle have combined spring store/diaphragm brake cylinders, the spring store parts of the combined spring store/diaphragm brake cylinders can be aerated with the reservoir pressure output for the trailer vehicle and provided by the parking brake modulator and can be deaerated. In the event of a drop in the reservoir pressure on the trailer vehicle, a parking brake on the trailer can be automatically engaged as a result.
A tractor vehicle in a vehicle combination often has a tractor vehicle protection valve. By means of the tractor vehicle protection valve, a compressed air line that provides an output pressure for the service brake of the trailer vehicle can be automatically shut off if the reservoir pressure provided for the trailer vehicle falls below a predetermined threshold value. For this purpose, use is conventionally made of a pneumatic holding valve. Without a holding valve of this type, a leaking compressed air line, for example, which provides an output service brake pressure to a trailer vehicle, might lead not only to a failure of the service brake function of the trailer vehicle, but, the output pressure for the service brake of the tractor vehicle could also drop. As a result, it might no longer be possible for the tractor vehicle to be braked in the desired manner.
The tractor vehicle protection valve prevents the output air pressure for the service brake of the tractor vehicle from dropping in the event of a pressure drop of the output air pressure for the reservoir pressure of the trailer vehicle. Here, it is assumed that compressed air possibly escapes both from the compressed air line that provides compressed air to the trailer vehicle for the service brake and also from the compressed air line that provides an output reservoir pressure to the trailer vehicle. This is, for example, the case if the trailer vehicle becomes detached from the tractor vehicle and, in the process, the two pressure lines that connect the tractor vehicle and trailer vehicle are torn away.
The compressed air line that provides a reservoir pressure to the trailer vehicle can then, if appropriate, be shut off from the compressed air reservoir of the tractor vehicle by means of an automatically or manually actuatable valve.
The known, separated design of parking brake modulator and tractor vehicle protection valve has the disadvantage that the tractor vehicle protection valve and at least one compressed air line that connects the tractor vehicle protection valve to the parking brake modulator must be installed separately in addition to the parking brake modulator. This leads to higher costs in the production of the components and during the installation of the components, and to a higher susceptibility to failure on account of the high number of compressed air lines, which can develop leaks. This results in higher costs for servicing and repair and higher costs on account of down time of the vehicle while repairs are carried out.